When is quarantine a useful control strategy for emerging infectious diseases?

Abstract

The isolation and treatment of symptomatic individuals, coupled with the quarantining of individuals that have a high risk of having been infected, constitute two commonly used epidemic control measures. Although isolation is probably always a desirable public health measure, quarantine is more controversial. Mass quarantine can inflict significant social, psychological, and economic costs without resulting in the detection of many infected individuals. The authors use probabilistic models to determine the conditions under which quarantine is expected to be useful. Results demonstrate that the number of infections averted (per initially infected individual) through the use of quarantine is expected to be very low provided that isolation is effective, but it increases abruptly and at an accelerating rate as the effectiveness of isolation diminishes. When isolation is ineffective, the use of quarantine will be most beneficial when there is significant asymptomatic transmission and if the asymptomatic period is neither very long nor very short.

FIGURE 1.

A schematic diagram of the two pathways down which an infected individual can move when quarantine protocols are in place.

FIGURE 2.

The effect of the reproduction number in the presence of isolation, R̄_I, on the efficacy of quarantine, as shown by the total expected number of infections averted by quarantine during an epidemic for each initially infected person (a) and the percentage of infections averted by quarantine during an epidemic (b). The parameter combination qρ is an overall measure of the effectiveness of trying to place an individual into quarantine, q is the probability that an asymptomatic individual will be identified and placed into quarantine, and ρ is the proportion of infections generated by an individual that can be prevented by placing the individual into quarantine (which is roughly equal to the proportion of infections generated by asymptomatic individuals).

FIGURE 3.

The mean number of infections averted by quarantine (solid curves), along with the mean plus 2 standard deviations (dashed curves). We expect at least 80 percent of the outcomes to lie below the bounds given by the dashed curves (Web appendix 2). Bold curves: σ_I = 0.5, σ_QI = 0.25, ρ = 0.05; thin curves: σ_I = 0.5, σ_QI = 0.25, ρ = 0.95. All results assume that q = 1.